Ultrasound is widely used for imaging a patient's internal structures without risk of exposure to potentially harmful radiation, as may occur when using X-rays for imaging. An ultrasound examination is a safe diagnostic procedure that uses high frequency sound waves to produce an image of the internal structures of a patient's body. Many studies have shown that these sound waves are harmless and may be used with complete safety, even to visualize the fetus in pregnant women, where the use of X-rays would be inappropriate. Furthermore, ultrasound examinations generally require less time than examinations using other imaging techniques, and ultrasound examinations are typically less expensive than examinations using other imaging techniques.
More recently, the use of high intensity focused ultrasound (HIFU) for therapeutic purposes, as opposed to imaging, has received significant attention in the medical community. HIFU therapy employs ultrasound transducers that are capable of delivering 1,000-10,000 W/cm2 to a focal spot, in contrast to diagnostic imaging ultrasound, where intensity levels are usually below 0.1 W/cm2. A portion of the energy from these high intensity sound waves is transferred to a targeted location as thermal energy. The amount of thermal energy thus transferred can be sufficiently intense to cauterize undesired tissue, or to cause necrosis of undesired tissue (by inducing a temperature rise greater than about 70° C.) without actual physical charring of the tissue. Tissue necrosis can also be achieved by mechanical action alone (i.e., by cavitation that results in mechanical disruption of the tissue structure). Further, where the vascular system supplying blood to an internal structure is targeted, HIFU can be used to induce hemostasis. The focal region of this energy transfer can be tightly controlled so as to obtain necrosis of abnormal or undesired tissue in a small target area without damaging adjoining normal tissue. Thus, deep-seated tumors can be destroyed with HIFU without surgical exposure of the tumor site.
An important component in any type of ultrasound therapy system is the mechanism for coupling the acoustic energy into the tissue. Good acoustic coupler is necessary to efficiently transfer the ultrasound energy from the transducer to the treatment site. The ideal acoustic coupler is a homogenous medium that has low attenuation, and an acoustic impedance similar to that of the tissue being treated. Due to its desirable acoustic transmission characteristics, water has commonly been used as the coupling medium in many therapeutic applications of ultrasound.
In previous hemostasis studies in which HIFU has been used to arrest bleeding of injured blood vessels and organs, the HIFU transducer was contained within a water-filled, conical, plastic housing with a thin, polyurethane membrane at the tip. This coupler was designed for superficial treatments, since it places the HIFU focus only several millimeters beyond the tip of the water-filled cone. While this coupling method has been useful for hemostasis experiments, it has many drawbacks that would make it impractical for a clinical setting. These disadvantages include requirements for degassing, sterilization, and circulation and containment issues. Due to the limitations of the current HIFU applicators, an alternative coupling medium is desirable.
Latex condoms have been used as a disposable sheath for rectal and vaginal ultrasound probes, and when filled with water, such sheaths facilitate acoustic coupling. With respect to HIFU therapy probes, a HIFU transducer can generate significant amounts of heat, which should be dissipated to protect the patient and to prolong the life of the transducer. Latex condoms, and ultrasound probe sheaths specifically intended for such ultrasound coupling, are not designed to facilitate circulation of a cooling liquid.
It would be desirable to provide a disposable acoustic coupler for use with ultrasound probes, configured to circulate a cooling liquid proximate an ultrasound transducer.